This invention relates to a speed control apparatus for an internal combustion engine of an automobile. More particularly, it relates to a speed control apparatus which can prevent fluctuations of the idle speed of an engine when electrical equipment of an automobile is switched on or off.
It is desirable to maintain the idle rotational speed of an automotive internal combustion engine at an optimal value in order to minimize engine noise, vibrations, and fuel consumption. Various feedback systems exist for controlling the idle speed of an engine. In these systems, an air bypass passage is provided which enables intake air to bypass the throttle valve. The air intake rate through the bypass passage is controlled by means of a control valve so as to minimize the difference between a target idle speed and the actual idle speed.
Due to delays in the detection of the rotational speed of an engine and response delays by the control valve for the air bypass passage, it takes time for a conventional rotational speed controller to adjust the idle speed to the target value. When electrical equipment of an automobile, such as headlights or fan motors, is turned on or off, the load applied to the engine by the generator which powers the electrical equipment suddenly changes. On account of the response delay of conventional speed controllers, the change in the engine load causes a momentary fall or rise in the engine speed when the electrical equipment is turned on or off, respectively.
Japanese Published Unexamined Patent Application No. 59-155547 discloses an idle speed control method for an automobile engine in which the operating state (on or off) of each piece of electrical equipment in an automobile is monitored by a corresponding sensor. When a piece of electrical equipment is switched on, the air intake rate into the engine is increased by a prescribed amount by opening a valve in an air bypass passage which bypasses the throttle valve of the engine. Similarly, when a piece of electrical equipment is switched off, the air intake rate through the air bypass passage is decreased. The increase or decrease in the air intake rate compensates for the increase or decrease in the load on the engine when the electrical equipment is turned on or off, thereby theoretically preventing a change in the engine rotational speed. The amount by which the air intake rate needs to be changed for the operation of each piece of electrical equipment is stored in a map in the memory of a control unit.
However, the above-described control method has the following drawbacks.
(1) An automobile is equipped with many different pieces of electrical equipment. If the electronic control unit is responsive to the switching on or off of each piece of equipment, a large number of sensors are necessary for detecting the operating state of the electrical equipment. Furthermore, the electronic control unit must have a large memory and large processing capacity in order to handle the input signals corresponding to all the pieces of electrical equipment. The electronic control unit therefore ends up being expensive and complicated.
(2) The data which is stored in the memory of the control unit indicates the average change in the air intake rate necessary to maintain a constant rotational speed when each piece of electrical equipment is turned on or off. For example, the memory contains the average change in the air intake rate corresponding to the operation of a typical set of headlights, a typical set of windshield wipers, etc. However, due to manufacturing inconsistencies, the properties of the electrical equipment which is actually mounted on a vehicle are often different from the properties of typical electrical equipment of the same type. Therefore, the necessary change in the air intake rate upon the operation of the headlights of a vehicle may be different from the average value stored in the memory of the electronic control unit. Furthermore, the extent to which a piece of electrical equipment actually acts as a load on an engine depends on a number of factors which are not taken into consideration by the data stored in the control unit, such as the engine operating temperature. Therefore, the change in the air intake rate when a piece of equipment is operated as indicated by the data in the memory may be different from the actual change in air intake rate necessary to maintain a constant engine speed.
(3) When a plurality of pieces of electrical equipment are simultaneously operated, the total change in the air intake rate required to maintain a constant engine speed may be less than a simple sum of the changes in air intake rate when each piece of equipment is operated individually. This is because the actual load which is applied to an engine when electrical equipment is operated is determined by the current which is output by the generator which powers the electrical equipment. The generator has a maximum generating capacity. If the total current demand from the various pieces of electrical equipment exceeds this generating capacity, the excess current demand is supplied by the battery of the vehicle and does not represent a load on the engine. If the air intake rate is increased in accordance with the total current demand by the electrical equipment, when the total current demand exceeds the generating capacity of the generator, the change in air intake rate will be excessive and the engine rotational speed will momentarily rise when the electrical equipment is turned on. In Japanese Published Unexamined Patent Application No. 59-155547 which is described above, an attempt is made to solve this problem by setting an upper limit on the increase in air intake rate. However, due to the impossibility of predicting the exact operating characteristics of a specific generator or of a specific piece of electrical equipment, as described in paragraph (2), in actual practice, it is impossible to set an accurate upper limit on the increase in the air intake rate, so the change in the air intake rate when electrical equipment is operated may be too small or too large.
(4) The electrical equipment of an automobile includes items such as turn signals and hazard lamps which draw a periodic current rather than a steady one. These items therefore exert a periodic load on an engine. To prevent the engine speed from fluctuating due to this load, it is necessary to adjust the air intake rate in a cyclic manner. As a result, the structure of the air intake rate controller becomes complicated. In addition, after a change in the setting of the valve in the air bypass passage is made, the engine must pass through the suction, compression, power, and exhaust strokes before the air intake rate actually changes. A surge tank for suppressing fluctuations in the air intake rate produces a further delay in the response of the actual air intake rate. The total delay due to these factors is referred to as the suction delay. If the period of the rise and fall of the current consumed by the electrical equipment is close in value to the suction delay, the changes in the air intake rate can become out of phase with the fluctuations in the current for which they are supposed to compensate. In this case, the engine rotational speed ends up being decreased when the electrical current is increasing, and it ends up being increased when the electrical current is decreasing. Instead of fluctuations in the engine rotational speed being suppressed, they are magnified, resulting in unstable engine operation.